TV magic, plain an simple. Clyde's adoptive mother starts a police record of her own during her first solo case, which also lands her the first intervention since she switched careers. Fortunately for the tortoise, Clyde's adoptive father is nothing but understanding. Sherlock has nothing but faith in Joan, and he would take no less from her.
Indeed Watson solves, not only her case, but Sherlock's as well once she discovers the cases, much like their respective investigators, are perfectly intertwined.